Mesocosm electromotive potential and pore network data

Summary of findings: We found that volume-based XCT-metrics were more frequently correlated with metrics describing changes in available energy than medial-axis XCT-metrics. Abundance of significant correlations between pore network metrics and available energy parameters was not only a function of pore architecture, but also of the dimensions of the sub-sample chosen for XCT analysis. Pore network metrics had the greatest power to statistically explain changes in available energy in the smallest volumes analyzed. Our work underscores the importance of scale in observations of natural systems. Two major objectives: 1.) Constrain the size of the soil volume that is "seen" by the tip of a Pt electrode 2.) Find quantitative, numerical indices of soil structure that can be used to test assumptions about causality regarding soil structure - redox state relationships Hypotheses: H1: Electromotive potentials sensed by platinum electrodes respond to changes in water saturation level in a predictable, non-random fashion. We hypothesize that for all electrodes probing the same network architecture type (i.e. sieving treatment), the associated metrics to characterize the available energy of reactions involving electron transfer (AE, parameterized as ΔEPt per time interval and expressed as fraction (%) of the free energy released by the oxidation of CH2O with O2) is more or less constant H2: The ability of XCT-derived pore network metrics to predict AE-metrics improves with decreasing average pore size, or ΔEPt,(a...j) = f(PNA), (PNM 1....18), where PNA = pore network architecture, (a...j) are a set of available energy metrics, and (1...18) is a set of pore network metrics H3: The ability of XCT-derived pore network metrics to predict AE-metrics improves when the volume of the observed pore network is small and immediately surrounds the platinum electrode tip, compared to larger soil volumes or 'the power of XCT derived network metrics to predict change in redox state = f(VoI) H4: AE-metrics are specific to pore network architectures. When subjected to the same moisture changes in AE-metrics should be significantly different between pore network architectures. This hypothesis can be accepted if ΔEPt per time interval ≠ΔEPt between pore network types Electromotive potential curve vs. Pore metrics: Electromotive potential data compared against pore network data derived from XCT measurements of the soil pore network within 9 mesocosms. Relative EPt calculation: Relationship derived to convert electromotive potential data (for each major terminal electron acceptor in soil systems) to energy available on a per electron transferred basis (= available energy) in kJ. Greenhouse electromotive potentials: Electromotive potential data collected from 27 Pt electrodes installed in 9 PVC mesocosms filled with soil from a Mollisol A horizon (Woodburn series).研究结果概要：本研究发现，基于体积的X射线计算机断层扫描（X-ray Computed Tomography, XCT）指标与描述可用能量变化的指标的相关性，显著高于沿中轴线的XCT指标。孔隙网络指标与可用能量参数间存在大量显著相关性，这一现象不仅取决于孔隙结构特征，同时也与XCT分析所选子样本的尺寸密切相关。在所分析的最小体积样本中，孔隙网络指标对可用能量变化的统计学解释能力最强。本研究凸显了自然系统观测中尺度效应的重要性。 本研究的两大核心目标：1. 限定铂（Platinum, Pt）电极尖端所探测的土壤体积范围；2. 挖掘可用于验证土壤结构与氧化还原状态之间因果关系假设的定量数值化土壤结构指标。 假设部分： 假设1：铂电极感应的电动势会以可预测、非随机的方式响应土壤水分饱和度的变化。我们提出假设：对于探测同一类孔隙网络结构（即经过相同筛分处理）的所有电极，表征涉及电子转移反应的可用能量的相关指标（AE，以单位时间间隔内的ΔEPt进行参数化，并表示为以O₂氧化CH₂O所释放自由能的百分比（%））大致保持恒定。 假设2：XCT衍生的孔隙网络指标预测AE指标的能力，会随平均孔径的减小而提升，对应关系式为ΔEPt,(a…j) = f(PNA, PNM₁…PNM₁₈)，其中PNA为孔隙网络结构（Pore Network Architecture, PNA），(a…j)为一组可用能量指标，(1…18)为一组孔隙网络指标。 假设3：相较于更大体积的土壤样本，当观测的孔隙网络体积较小且紧邻铂电极尖端时，XCT衍生的孔隙网络指标预测AE指标的能力将显著增强，即“XCT衍生网络指标预测氧化还原状态变化的能力 = f(VoI)”（VoI为感兴趣体积，Volume of Interest）。 假设4：可用能量（AE）指标具有孔隙网络结构特异性。当经历相同的水分变化条件时，不同孔隙网络结构对应的AE指标应存在显著差异。若不同孔隙网络类型间的单位时间间隔ΔEPt存在显著差异，则该假设成立。 电动势曲线与孔隙指标：将电动势数据与通过XCT测量9个中型土壤生态箱（mesocosms）内土壤孔隙网络得到的孔隙网络数据进行对比分析。 相对EPt计算：推导得到将电动势数据（针对土壤系统中的各主要末端电子受体）转换为每转移电子所对应可用能量（单位：kJ）的定量关系式。 温室电动势数据：从安装于9个聚氯乙烯（Polyvinyl Chloride, PVC）中型土壤生态箱的27个铂电极上采集得到电动势数据，这些生态箱填充取自软土（Mollisol）A层（Woodburn系列）的土壤。